Foucault's Pendulum, 1851

"That was when I saw the Pendulum. The sphere, hanging from a long wire set into the ceiling of the choir, swayed back and forth with isochronal majesty.. .The time it took the sphere to swing from end to end was determined by an arcane conspiracy between the most timeless of measures: the singularity of the point of suspension, the duality of the plane's dimensions, the triadic beginning ofn, the secret quadratic nature of the root, and the unnumbered perfection of the circle itself... Were its tip to graze, as it had in the past, a layer of damp sand spread on the floor of the choir, each swing would make a light furrow..."--Umberto Eco, Foucault's Pendulum.

I've uploaded an interesting recrod to the books for sale section of this blog on the great experiment of Leon Foucault (1819-1868), who was the first to actually demonstrate the rotation of the Earth, doing so with a very simple, extraordinarily elegant experiment involving a heavy brass bob suspended from a long cable--a pendulum that was unencumbered and free to swing along any plane. It is the curvature of the Earth that allows the tip of the bob to make its pattern, and it is the fact that the Earth is rotating under the moving pendulum that allows it to be tracing this path at all--it is also tells the difference between living on a sphere and living on a plane.

[The original experiment used a 5k bob hanging from a basement ceiling of about 2 meters; in just a bit, Foucault would move the experiment/demonstration to the Pantheon in Paris, using a 28k bob suspended from a 67 meter cable.]

Foucault, Jean Bernard Leon. "Physikalischer Beweis von der Axendrehung der Erde mittelst des Pendels". In Annalen der Physik, Leipzig, Johann Ambrosius Barth, 1851, volume 82. We offer the entire volume of the half-year, the Foucault paper occupying pp 458-462. This is the first appearance in German (following the French publication in the Comptes Rendus on 3 February 1851) of Foucault's great and marvelous experiment and proof of the rotation of the Earth. [Available here.]

Here's an animation of the movement of the pendulum

Animation of a Foucault pendulum at the Pantheon in Paris (48°52' North), with the Earth's rotation rate greatly exaggerated. The green trace shows the path of the pendulum bob over the ground (a rotating reference frame), while the blue trace shows the path in a frame of reference rotating with the plane of the pendulum. [Animated graphic via Wiki, here.]